The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
In many visual attitude determination systems, the shape of the object of interest must be accurately known, since the operation of creating a score to indicate the certainty of a visual attitude determination depends on a close match between the image of the object and a reference image of the object. However, real-world cases often involve objects which have some degree of ambiguity in their shape or silhouette. Examples of this are aircraft with detachable fuel tanks, satellites with articulating solar panels, or aircraft with variable external weapons stores. In these cases, the overall score for comparison of the object with the library image with the correct attitude will be reduced if the object's shape does not exactly match the library image. For example, FIG. 1A shows the binary image of an F-16 aircraft whose attitude is to be determined. However, this particular aircraft has external fuel tanks on the wings which the library image (FIG. 1B) does not have. When attempting to score the “fit” for these two images, even though the actual pitch, yaw, and roll do match exactly, the score is reduced by the mismatch of the additional fuel tanks, as indicated in FIG. 1C. Depending on the degree of mismatch, this could significantly affect the accuracy of the overall “Fit” score. In this case, the Fit score without the tank mismatch would be 100%, but with the mismatch, it would only be about 99.1%. This reduction in the Fit score could be significant for some attitude determination applications.
As shown in the example of an F-16 aircraft mentioned above, such an aircraft can have a relatively large number of different configurations, depending on the number of external fuel tanks and various combinations of ordnance mounted on the wings. Thus, it may not be practical to attempt to create a silhouette library using only one configuration of an F-16 which will be suitable for a particular application. One solution might be to create different libraries for every possible weapons stores/fuel tank combination. However, this may not be practical, since the attitude monitoring system and its software may not recognize a particular aircraft because of a unique weapons stores/fuel tank combination that is not included in one of the libraries. Furthermore, the number of libraries to choose from could be quite large due to the numbers of weapons stores/fuel tank combinations possible.
Other factors that can affect the silhouette of the object being imaged, and thus significantly complicate the task of accurately determining an attitude of the object, can range from minor visual impediments, such as with propeller rotation, to major conditions such as exhaust plume contamination. An exhaust plume can be larger and brighter than the aircraft itself, thus significantly affecting the ability of a visual attitude monitoring/determination system to determine the attitude of the aircraft. Such a severe condition may even prevent any meaningful determination of the object's attitude by the silhouette method, since scoring for a correct determination depends upon a satisfactory pixel-to-pixel match between the object being observed and the library view of the object.
From the foregoing, it should be appreciated that with two dimensional attitude determination of objects, situations may exist where the ability to resolve the attitude of an object is complicated because of structure carried on the object of interest, or external structure that occludes an image of the object of interest. In general, anything that interferes with the fundamental, static silhouette of the object can add significant difficulty into the operation of evaluating the attitude of the object.